replacing male, and turning the old male into female???

i have a breeding pair of onyx percula that lays regularly. i just got a small picasso that is about 9 month old.
im planning to replace the old male from the breeding pair and using the new picasso male. so i can breed some cool looking picassos. 
what do you guys think about this? any suggestion.
 
also, the old onyx male, im thinking of turning it into a female. the male is at least 3.5 years old,  how long do i have to wait, before it turn into a breeding female??? anyone has any experience??
thanks!

Not sure if my experience is representative, but I had a male fertilise a nest in May 2014. I had a juvenile ocellaris in the same tank and following the death of the female (jumped, or maybe pushed, out the tank) the two remaining fish paired up.
 
Yesterday they laid their first nest, and it looks like the eggs are all fertilised. So, for me, it was 9 months from fertilising the eggs to laying them.

I lost a bunch of females from pairs last July, and I couldn't find any mature females to replace them, so I paired the remaining males up with juveniles.  I guess I'm around 7-8 months.  They have all been cleaning the tiles for months already, but so far no eggs.   Interestingly, these "new pairs" have been very good at fostering eggs for me from pairs out of reef tanks, but recently the "new pairs" won't foster anymore.

Don't let fear and common sense stop you! =]

thanks for the info!
do you know the age of the old male? could it because the male was not old enough to be the female then took 9 month for it to be a breeding female?

according to your info and Magicj's, it looks like i will need to wait quiet awhile.
IMO if the pair is not  fostering, instead they eat all eggs. it could mean that they are going to lay soon.
 

 
Maybe, maybe not.  Fostering is hit or miss.  The new females don't look gravid yet.

Don't let fear and common sense stop you! =]

i tried the double down method with 3 pairs.... 2 pairs that ate all the eggs, laid within 2 weeks, the BW pair took good care of the eggs, but its been 2 month and they haven't lay me anything yet. but im not sure, maybe just coincidence？
 
i was looking at your post about the food blend. i have a few questions..
my WW pair laid another batch after i changed the food blend. all eggs are write with a "solid" look not crystal. (previous batches are all good)
my current food including:
Prawn, salmon, clam, squid, squid content, chicken egg, fish roe, seaweed
i was thinking it maybe because of the food, cuz, i had experience of feeding certain find of food like lobster causing all my fish to lay such kind of eggs.
what do you think???
 
 
 

 
No, I don't know the age of the old male - he had been in my tank for over 3 years so I don't think that was the problem. More likely was the fact that the new male was only 7 months old and was probably too immature. I also suspect that environmental factors would play a part of the sex change i.e. what other females are in the vicinity.